1. Field of the Invention
The present invention relates to a system and apparatus for detecting and reporting a movable body by using electromagnetic wave or radio communication.
2. Description of Related Art
A variety of conventional methods for detecting another approaching vehicle or another approaching movable body while the driver is driving a vehicle have been proposed so far.
For example, according to a method disclosed in Japanese Patent Laid-open No. Hei 2-216600, a vehicle is running while a receiver provided in the vehicle is monitoring incoming signals. Furthermore, a warning wave transmitted by a transmitter is installed on another movable body which is received by the receiver. A warning is then generated to notify the driver of the existence of the other movable body.
However, the number of movable bodies causing a warning is not known. Thus, in some cases, the existence of only one vehicle can be confirmed by the sense of eight or more vehicles. Therefore, it is difficult to verify the existence of other vehicles.
In order to solve this problem, there has been proposed a method as disclosed in Japanese Patent Laid-open No. Hei 7-225274. With this method, it is possible to determine whether only one movable body or a plurality of movable bodies exist within a predetermined distance by comparing an interval between rising edges of two consecutive pulses of a received signal and to display a result of the determination.
However, a moveable body detected may disappear from detection temporarily behind an obstruction such as another large-sized vehicle or a fence before appearing again.
In this case, when the movable body disappears behind the obstruction, the warning wave transmitted thereby can no longer be received, causing a warning generated by the wave to be halted. Thus, the driver may forget the existence of the movable body, being no longer alert.
In such a case, when the movable body disappearing from detection temporarily appears again, the movable body which no longer receives attention does reappear, generating a warning wave. As a result, the driver gets bewildered.
It is also difficult for the driver to immediately determine whether a warning is generated from a warning wave transmitted by a reappearing movable body or newly transmitted by another movable body.
By merely knowing the number of other movable bodies; however, the existence of a plurality of movable bodies still raises a problem in that when 2 or more vehicles can be verified by the sense of sight, it is quite within the bounds of possibility that another hidden vehicle still exists.
That is to say, it is impossible to form a judgment as to whether or not a vehicle exists behind an obstruction with a high degree of reliability.